A phantom is a device that simulates a body of tissue in its interaction with radiation. Various types of phantoms are used to test the performance of medical imaging equipment by mimicking the radiation attenuation and absorption properties of human tissue. Phantoms are also used to measure radiation dosage during therapy, for teaching purposes, to calibrate imaging equipment and for research. Phantoms are an essential element of maintaining high therapeutic and diagnostic quality assurance and control. When applied to image quality control, phantoms are used to measure system resolution, focal spot size, contrast, exposure controls, image artifacts, etc.
Two classes of phantoms are widely used. One type of phantoms uses plastics, gels, water and other chemical mixtures to simulate human tissue and organs. These devices are referred to as “tissue equivalent.” The other type of phantoms generates test patterns for confirming and evaluating system performance.
Numerous devices for marking, calibrating and aligning images from CAT and MRI systems are known in the art. For example, U.S. Pat. No. 5,299,253 of Wessels describes an alignment system and method to uniquely identify a cross-section of an imaged object to facilitate correlation of images. This is especially useful in identifying lesions near organs where a tumor may be obscured by an adjacent organ. It is, however, unrelated to the objective of this invention. U.S. Pat. No. 5,416,816 of Westrup describes a calibration template for computed radiography. It includes a variety of elements which simulate the X-ray absorption characteristics of various human body portions and organs. This device is useful for training radiologists and facilitates standardization of CAT image quality, which is especially useful for remote analysis of transmitted radiographic digital data. This device is of little use in assessing accuracy of table movements, however. U.S. Pat. No. 3,714,428 of Gasaway discloses a marker for radiology. A radiolucent member having stepped sloping edges with radio-opaque numerals is used to automatically record the height of the visible plane appearing on the film relative to a reference surface. The image of the numeral located closest to the plane is visible on the film while the remaining indicia are obscured.
Most MR imaging centers have some form of machine and image quality control that ensures that the acquired images are of sufficient quality for clinical evaluation. Test objects (“phantoms”) are often used for this purpose, and a number of investigators have described their application to determinations of the accuracy of volumetric measurements (Tofts et al., Magn Reson Imaging 1997; 15(2):183–92), resolution (Fellner et al. Magn Reson Imaging 2001 July; 19(6):899–904), and relaxation time measurements (Laubach et al., J. Magn. Reson. Imaging 1998 8(6):1349–54; Fellner et al., supra; and Kjaer et al., Acta Radiol. 1987 May–June; 28(3):345–51). Phantoms have also been used to determine the longitudinal (Firbank et al., Br. J Radiol. 2000 April; 73(868):376–83) and inter-site (Barker et al., Magn. Reson. Imaging 1992; 10(4):585–95) stability of many of these measures. Phantoms typically have been made of acrylic (Tofts; Firbank; and Fellner, supra), or other non-metallic materials (Tofts, supra; Luft et al., J. Magn. Reson. Imaging 1996 July–August; 6(4):700–4; Disler et al., Invest. Radiol. 1994 August; 29(8): 739–45; and Laubach et al., J. Magn. Reson. Imaging 1998 November–December; 8(6):1349–54), and filled with water (Tofts; and Disler, supra), aqueous solutions of paramagnetic ions (Tofts; Luft; Disler; Laubach; Firbank; Fellner; Kraft et al., Magn. Reson. Med. 1987 December; 5(6): 555–62; and Kjaer et al., Acta Radiol. 1987 May–June; 28(3):345–51), gels (Lufts; Laubach; Kraft; and Kjaer, supra), vegetable oil (Disler, supra), and other materials that produce an MRI signal. These phantoms incorporate a number of shapes and configurations depending upon their purpose. Among these are cylinders (Tofts; Luft; Disler; and Fellner, supra) cones (Firbank, supra; and Coffey et al., Med. Phys. 1989 March–April; 16(2): 273–8), spheres (Disler; and Firbank, supra), and irregular (Lufts; Disler and Lauback, supra) or deformable compartments constructed of, for example rubber (Disler; and Laubach, supra).
None of the prior phantoms are well suited for the assessment of quantitative measures of joint cartilage, especially because of they inadequately model cartilage curvature and thinness. Surprisingly, the present invention provides a phantom that is ideally suited for quality control of images of joints and joint cartilage.